38 SUMMARY OF CURRENT RESEARCHES RELATING TO 



hair which penetrates through the cuticle. In the other — the polyp- 

 cells — the fibril is long, displays various swellings, but always bears 

 an ampulla with a narrow neck at the peripheral end. From the neck 

 a number of secondary fibrils radiate like the tentacles of a polyp. 

 These do not perforate the cuticle, but spread out over its inner surface. 

 These cells are most abundant in the vicinity of the upper jaw. Where 

 a number occur together, the nuclei (cell-bodies) form a compact mass 

 from which nerve-fibre arises. It would appear that the polyp-cells 

 are connected with the buccal ganglion, and the sensory cells of the 

 lips with the cerebral ganglion. 



Nerve-cells of Limax.*— Dr. J. Havet has studied these by Golgi's 

 methods. The skin contains unipolar, bipolar, and multipolar cells. 

 The bipolar cells are branched externally, and form an abundant super- 

 ficial innervation. The ganglia consist of peripheral nerve-cells (uni- 

 polar, bipolar, and multipolar) and a punctate central substance. 

 Numerous clear figures illustrate the peculiarities of minute structure. 



Water-absorption in Slugs.f— Herr Karl Kiinkel has made a series 

 of experiments in order to determine the amount of water capable of 

 being taken up by Mollusca without shells, and the way in which it 

 is taken up. By experiments with Limax cinereus, the author proved, 

 first that water is absorbed, second that it can be taken through the skin, 

 or through both skin and mouth. Again, when dried up the snails lose 

 their power of movement, because the mucus becomes too thick to flow. 

 Such dried specimens can absorb sufficient water through the skin to 

 make the mucus-fluid, and then recover their power of movement. This 

 is the reason why the slugs leave their hiding places only in the evening 

 or after rain, and the grooves and ridges of the skin serve, the former 

 to retain the water, and the latter to conduct it over the surface. The 

 mucus itself is not hygroscopic, nor can the snails condense water even 

 from air saturated with vapour. The mucus does, however, absorb 

 water, and then greatly increases in volume and becomes fluid ; while 

 similarly on drying it diminishes in volume and becomes thick and 

 tenacious. There is some difference in the absorbent power of the 

 mucus of different species of slugs. It seems probable that the absorp- 

 tion of water by dried snails is due to the mucus within the pedal gland, 

 which probably absorbs water through the opening of the gland. 



Reversal of Cleavage in Ancylus4— Mr. S. J. Holmes finds that in 

 the " fresh-water limpet " (Ancylus rivulorum Say)— a sinistral form — 

 tuere is a reversal of the cleavage, like that described in Physa by 

 Crampton. The reversal shows its effects as late as the gastrula stage, 

 which lends considerable support to the view that it may stand in some 

 causal relation to the reversed asymmetry of the adult. As the author 

 points out, the reversal of cleavage in Ancylus has a special significance 

 from the fact that the left-handedness occurring in this genus has, in all 

 probability, arisen independently of that of Physa and Planorbis. It 

 will be of interest to inquire if the cleavage of the dextral species of 

 Ancylus (subgenus Acroloxus) is of the normal type. 



* Anat. Auzeig., xvi. (1890) pp. 241-S (10 figs.), 

 t Zool. Anzeig., xxii. (1899) pp. 388-96. 401-4. 

 % Araer. Nat., xxxiii. (1899) pp. 871-6. 



